Sprites: things that move without the screen moving

Everything drawn on this machine was in a cell. Something between two
cells meant rewriting both; something moving a pixel at a time meant
rewriting them sixty times a second, which is affordable for one thing
and not for twenty. A sprite is put at a pixel and the device draws it
over whatever is behind, so moving it costs two bytes.

MADE OF TILES, which is the decision the rest follows from: m by n taken
in reading order from one index, so there is no second pixel format, no
second kind of memory, and nothing a sprite can show that the map cannot.
A 16 by 16 character is four tiles and the background can name the same
four.

256 entries of 8 bytes at 0xC000 in the atlas - eight so the entry
address is a shift, the same no-multiply argument as the palette's four.
Position is signed and sixteen bits, because 640 by 400 does not fit in a
byte and a sprite has to be able to sit half off the left rather than
appearing whole at the edge.

A PIXEL OF ZERO IS NOT DRAWN, or every sprite is a rectangle. Tested
before the attribute is added, so a hole belongs to the art and not to
the colour scheme. The same rule the other way round is what "behind"
means: drawn only where the background pixel was zero, so a thing walks
behind a pillar and in front of the floor in one frame.

All of them draw, every frame, so they cannot flicker. Real machines
dropped them per scanline because they had a fixed number of shift
registers; this has a loop. The limit is the size of the table, which is
a constant rather than a property of what is on screen.

And the system takes them down at exit. The sprite table sits in the gap
the screen save walks around - to the end of the map, then the palette -
and that is right, because nothing the shell draws is a sprite: there is
nothing to give back, only something to take away. Otherwise a program
that put a ball up and left would leave it over the prompt, in front of
everything, with nothing able to type it away. Sprite.asm deliberately
leaves its own, because a program that faulted could not have cleared it.

Every check here was re-broken and failed: transparency, reading order,
draw order, priority, and size. Size needed breaking twice - the first
attempt did not compile, and a silent build failure had left the old
binary passing.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E2JrLzFvuFX9fgi1LDRjrW

Tests/expected/cosmosCrossDisk.out

@@ -23,6 +23,7 @@ Grid.sbx                571
 Press.sbx               872
 Mode.sbx                48
 Flip.sbx                173
+Sprite.sbx              442
 Crash.sbx               632
 vars.script             50
 blocks.script           343
@@ -39,7 +40,7 @@ outer.script            376
 inner.script            44
 loop.script             35
 crossed.txt             560
-26 files, 1 directory
+27 files, 1 directory
 > exit
 halted
 Execution halted.

Tests/expected/cosmosDrives.out

@@ -13,6 +13,7 @@ Grid.sbx                571
 Press.sbx               872
 Mode.sbx                48
 Flip.sbx                173
+Sprite.sbx              442
 Crash.sbx               632
 vars.script             50
 blocks.script           343
@@ -28,7 +29,7 @@ nonl.script             38
 outer.script            376
 inner.script            44
 loop.script             35
-25 files, 1 directory
+26 files, 1 directory
 > drive 1
 > dir
 other.txt               28

Tests/expected/cosmosFault.out

@@ -30,6 +30,7 @@ Grid.sbx                571
 Press.sbx               872
 Mode.sbx                48
 Flip.sbx                173
+Sprite.sbx              442
 Crash.sbx               632
 vars.script             50
 blocks.script           343
@@ -45,7 +46,7 @@ nonl.script             38
 outer.script            376
 inner.script            44
 loop.script             35
-25 files, 1 directory
+26 files, 1 directory
 > exit
 halted
 Execution halted.

Tests/expected/cosmosFlip.out

@@ -20,6 +20,7 @@ Grid.sbx                571
 Press.sbx               872
 Mode.sbx                48
 Flip.sbx                173
+Sprite.sbx              442
 Crash.sbx               632
 vars.script             50
 blocks.script           343
@@ -35,7 +36,7 @@ nonl.script             38
 outer.script            376
 inner.script            44
 loop.script             35
-25 files, 1 directory
+26 files, 1 directory
 > exit
 halted
 Execution halted.

Tests/expected/cosmosGrid.out

@@ -20,6 +20,7 @@ Grid.sbx                571
 Press.sbx               872
 Mode.sbx                48
 Flip.sbx                173
+Sprite.sbx              442
 Crash.sbx               632
 vars.script             50
 blocks.script           343
@@ -35,7 +36,7 @@ nonl.script             38
 outer.script            376
 inner.script            44
 loop.script             35
-25 files, 1 directory
+26 files, 1 directory
 > exit
 halted
 Execution halted.

Tests/expected/cosmosMonitor.out

@@ -106,6 +106,7 @@ Grid.sbx                571
 Press.sbx               872
 Mode.sbx                48
 Flip.sbx                173
+Sprite.sbx              442
 Crash.sbx               632
 vars.script             50
 blocks.script           343
@@ -121,7 +122,7 @@ nonl.script             38
 outer.script            376
 inner.script            44
 loop.script             35
-25 files, 1 directory
+26 files, 1 directory
 > exit
 halted
 Execution halted.

Tests/expected/cosmosMonitorRun.out

@@ -25,6 +25,7 @@ Grid.sbx                571
 Press.sbx               872
 Mode.sbx                48
 Flip.sbx                173
+Sprite.sbx              442
 Crash.sbx               632
 vars.script             50
 blocks.script           343
@@ -40,7 +41,7 @@ nonl.script             38
 outer.script            376
 inner.script            44
 loop.script             35
-25 files, 1 directory
+26 files, 1 directory
 > exit
 halted
 Execution halted.

Tests/expected/cosmosRun.out

@@ -13,6 +13,7 @@ Grid.sbx                571
 Press.sbx               872
 Mode.sbx                48
 Flip.sbx                173
+Sprite.sbx              442
 Crash.sbx               632
 vars.script             50
 blocks.script           343
@@ -28,7 +29,7 @@ nonl.script             38
 outer.script            376
 inner.script            44
 loop.script             35
-25 files, 1 directory
+26 files, 1 directory
 > load
 load what?
 > load nosuch.sbx

Tests/expected/cosmosSlowDisk.out

@@ -11,6 +11,7 @@ Grid.sbx                571
 Press.sbx               872
 Mode.sbx                48
 Flip.sbx                173
+Sprite.sbx              442
 Crash.sbx               632
 vars.script             50
 blocks.script           343
@@ -26,7 +27,7 @@ nonl.script             38
 outer.script            376
 inner.script            44
 loop.script             35
-25 files, 1 directory
+26 files, 1 directory
 > load Say.sbx
 loaded, starting at 5000
 > run the disk took its time

Tests/expected/cosmosSprite.out

@@ -0,0 +1,43 @@
+CosmOS
+> Say before Sprite
+it says: before Sprite
+finished
+> Sprite
+A ball, over the shell's own words. Nothing underneath is written to. Press a key.
+finished
+> Say after Sprite
+it says: after Sprite
+finished
+> dir
+greet.sbx               211
+hello.sbx               53
+Life.sbx                1396
+Snake.sbx               2164
+Keys.sbx                664
+Say.sbx                 156
+Break.sbx               149
+Grid.sbx                571
+Press.sbx               872
+Mode.sbx                48
+Flip.sbx                173
+Sprite.sbx              442
+Crash.sbx               632
+vars.script             50
+blocks.script           343
+loops.script            272
+tune.sbx                306
+notes.txt               21
+Apps                    <dir>
+hi.script               121
+bad.script              45
+plain.script            24
+cross.script            280
+nonl.script             38
+outer.script            376
+inner.script            44
+loop.script             35
+26 files, 1 directory
+> exit
+halted
+Execution halted.
+[exit 0]